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    Universal Serial Bus

    Unlike other serial interface technologies such as RS232 that is free to use, USB is not. If you want to design a USB product then you have to pay a 1500$ fee each year for getting ID numbers to use with your USB device for operational. http://www.usb.org

    USB got four wires.

    +3.3v
    
    D+
    
    D-
     
    GND
    

    The USB hardware select between full speed usb2.0 and lowspeed usb1.0 by examine if any of the data wires D+ or D- got a pullup resistor connected.

    1.5Kohm connected to D+ from 3.3V = fullspeed USB

    1.5Kohm connected to D- from 3.3V = lowspeed USB

    The USB-host is a little dictator that gives each device a slice of each juicy 1ms timeslot that is distributed to the USB-devices.


    USB-devices can be powered from the USB cable, if the USB-device feel suicidial when connected then it can ask if the USB-host wants to send over some go-up-in-smoke frying current. (500 mA max)


    HackerPort - IO-Pins via USB for hardware hacking

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